Since OS X Mountain Lion, the Mac defaults to preventing applications from unidentified developers or sources from being launched. You’ll discover the message in OS X 10.8 when you try to launch a Mac app that didn’t come from a verified source or from the Mac App Store, and you’ll get an alert dialog that says “[App name] can’t be opened because it is from an unidentified developer”.
This new security feature is called GateKeeper, and it doesn’t mean you can’t run those unverified apps on the Mac, you just have to either temporarily skirt the security blanket of GateKeeper, or turn off the app limitations entirely.
Temporarily Get Around “App Can’t Be Opened” Alert Message
1. Right-click (or control-click) the application in question and choose “Open”
2. Click the “Open” button at the next dialog warning to launch the app anyway
Disable GateKeeper’s Unidentified App Developer Prevention Completely
1. Launch System Preferences from the Apple menu
2. Choose “Security & Privacy” and then click the “General” tab, followed by clicking the lock icon in the corner to unlock the settings
3. Look for “Allow applications downloaded from:” and choose “Anywhere”
4. Accept the security warning and allow
5. You can now launch any app from any location or developer